Electroluminescent (EL) devices have been known for some years and have been recently used in commercial display devices. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of subpixels. Each subpixel contains an EL emitter and a drive transistor for driving current through the EL emitter. The subpixels are typically arranged in two-dimensional arrays with a row and a column address for each subpixel, and having a data value associated with the subpixel. Subpixels of different colors, such as red, green, blue and white, are grouped to form pixels. EL displays can be made from various emitter technologies, including coatable-inorganic light-emitting diode, quantum-dot, and organic light-emitting diode (OLED).
OLED displays are of particular interest as a superior flat-panel display technology. These displays utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Different organic materials emit different colors of light. However, as the display is used, the organic materials in the display age and become less efficient at emitting light. This reduces the lifetime of the display. The differing organic materials can age at different rates, causing differential color aging and a display whose white point varies as the display is used. In addition, each individual pixel can age at a rate different from other pixels, resulting in display nonuniformity. Further, some circuitry elements, e.g. amorphous silicon transistors, are also known to exhibit aging effects.
The rate at which the materials age is related to the amount of current that passes through the display and, hence, the amount of light that has been emitted from the display. Various techniques to compensate for this aging effect have been described.
U.S. Pat. No. 6,414,661 B1 by Shen et al. describes a method and associated system to compensate for long-term variations in the light-emitting efficiency of individual organic light-emitting diodes (OLEDs) in an OLED display by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel. The method derives a correction coefficient that is applied to the next drive current for each pixel. This technique requires the measurement and accumulation of drive current applied to each pixel, requiring a stored memory that must be continuously updated as the display is used, and therefore requiring complex and extensive circuitry.
U.S. Pat. No. 6,504,565 B1 by Narita et al. describes a similar method of holding the amount of light emitted from each light-emitting element constant. This design requires the use of a calculation unit responsive to each signal sent to each pixel to record usage, greatly increasing the complexity of the circuit design.
U.S. Patent Application Publication No. 2002/0167474 A1 by Everitt describes a pulse width modulation driver for an OLED display. One embodiment of a video display comprises a voltage driver for providing a selected voltage to drive an organic light-emitting diode in a video display. The voltage driver can receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics. In one embodiment of the invention, the correction tables are calculated prior to and/or during normal circuit operation. Since the OLED output light level is assumed to be linear with respect to OLED current, the correction scheme is based on sending a known current through the OLED diode for a duration sufficiently long to permit the transients to settle out, and then measuring the corresponding voltage with an analog-to-digital converter (A/D) residing on the column driver. A calibration current source and the A/D can be switched to any column through a switching matrix.
JP 2002-278514A by Numao describes a method in which current through and temperature of organic EL elements are measured. Compensation is then performed using precomputed tables and the current and temperature measurements. This design presumes a predictable relative use of pixels and does not accommodate differences in actual usage of groups of pixels or of individual pixels. Hence, correction for color or spatial groups is likely to be inaccurate over time. Moreover, the integration of temperature and multiple current sensing circuits within the display is required. This integration is complex, reduces manufacturing yields, and takes up space within the display.
U.S. Patent Application Publication No. 2003/0122813 A1 by Ishizuki et al. discloses a method which measures current for each subpixel in turn. The measurement techniques of this method are iterative, and therefore slow.
U.S. Pat. No. 6,995,519, by Arnold et al., teaches a method of compensating for aging of an OLED emitter. This method assumes that the entire change in device luminance is caused by changes in the OLED emitter. However, when the drive transistors in the circuit are formed from amorphous silicon (a-Si), this assumption is not valid, as the threshold voltage of the transistors also changes with use. This method will not provide complete compensation for OLED efficiency losses in circuits wherein transistors show aging effects. Additionally, when methods such as reverse bias are used to mitigate a-Si transistor threshold voltage shifts, compensation of OLED efficiency loss can become unreliable without appropriate tracking/prediction of reverse bias effects, or a direct measurement of the OLED voltage change or transistor threshold voltage change.
U.S. Patent Application Publication No. 2004/0100430 A1 by Fruehauf discloses a pixel structure having a third transistor which taps a diode driving current to supply a current-measuring circuit and a voltage comparison unit. However, this method reduces the efficiency of a display containing such pixels by using for measurement current which could have otherwise been used to emit light. Furthermore, this method only compensates for TFT variations and is unable to compensate for non-uniform OLED characteristics.
In addition to aging effects, some transistor technologies, such as low-temperature polysilicon (LTPS), can produce drive transistors that have varying mobilities and threshold voltages across the surface of a display (Kuo, Yue, ed. Thin Film Transistors: Materials and Processes, vol. 2: Polycrystalline Thin Film Transistors. Boston: Kluwer Academic Publishers, 2004, pg. 410-412). This produces objectionable visible nonuniformity. Further, nonuniform OLED material deposition can produce emitters with varying efficiencies, also causing objectionable nonuniformity. These nonuniformities are present at the time the panel is sold to an end user, and so are termed initial nonuniformities. FIG. 9 shows an example histogram of subpixel luminance for a flat field exhibiting differences in characteristics between pixels. Actual luminances varied by 20 percent in either direction, resulting in unacceptable display performance.
U.S. Pat. No. 6,081,073 by Salam describes a display matrix with a process and control circuitry for reducing brightness variations in the pixels. This disclosure describes the use of a linear scaling method for each pixel based on a ratio between the brightness of the weakest pixel in the display and the brightness of each pixel. However, this approach will lead to an overall reduction in the dynamic range and brightness of the display and a reduction and variation in the bit depth at which the pixels can be operated.
U.S. Pat. No. 6,473,065 B1 by Fan describes methods of improving the display uniformity of an OLED. The display characteristics of all organic-light-emitting-elements are measured. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction. However, this method requires optical measurements. This makes it unsuitable for aging correction, which requires periodic measurement in the user's location. Further, the described approaches require either a separate lookup table for each pixel, resulting in very expensive memory requirements, or approximations to the characteristics of each pixel, reducing image quality.
U.S. Patent Application Publication No. 2005/0007392 A1 by Kasai et al. describes an electro-optical device that stabilizes display quality by performing correction processing corresponding to a plurality of disturbance factors, and using a conversion table whose description contents include correction factors. However, this method requires a large number of look-up tables (LUTs), not all of which are in use at any given time, to perform processing, and does not describe a method for populating those LUTs.
There is a need therefore for a more complete compensation approach for aging and initial nonuniformity of electroluminescent displays.